1. Field of the Invention
The present invention relates to material deposition. More particularly, the invention relates to material hydride vapour phase epitaxy (HVPE) deposition using a vertical growth cell with extended diffusion and time modulated growth processes for growth of semiconductors of IV and III-V groups and their alloys.
2. Description of the Prior Art
Many processes have been developed for the deposition of materials, e.g. semiconductor materials, on a substrate. One such process involves the use of a precursor gas, i.e. a gas that upon contact with the substrate undergoes a modification such as a chemical reaction to yield a deposited layer. In these vapour deposition processes, generally, the gas flow and its spatial relationship to the substrate are carefully controlled. For example, in the most common spatial configuration employed in chemical vapour deposition (CVD) or hydride vapour phase epitaxy (HVPE), as shown in FIG. 1, a gas flow is established at one end of a horizontal vessel 1, a substrate 3 is placed within the vessel, and a gas flow is established in the direction of arrows 2, parallel to the major surface of the substrate 3. There are many disadvantages for such horizontal reactors. The main drawback is the difficulty to obtain simultaneously high efficiency of gas utilization and good uniformity of growth. Generally, the uniformity δ of a rotating substrate can be approximated by the equation:δ=η(1−2/π)/(1−η), where η is the efficiency of gas utilization.
This means that for a uniformity better than 3%, the deposition efficiency will be less than 10%. The symmetry and the related flow dynamics make the growth processes difficult to control, and also make the reactor difficult to scale up.
In all alternative configuration shown in FIG. 2 employed in vertical CVD or HVPE processes, the gas flow direction 2 is generally perpendicular to the major surface of the substrate 3. The first configuration, i.e. the horizontal configuration, is most commonly used because it introduces the least perturbation in the precursor gas flow. However, the latter configuration is at times employed when it is desired to minimize the temperature gradient across the substrate introduced by the corresponding axial temperature gradient in the reactor. Disadvantages of conventional vertical tube HVPE reactors include the difficulty of controlling the growth uniformity due to its simple cylindrical tube configuration, and the large amount of parasitic deposition on reactor walls resulting from the difficulties in obtaining controlled temperature difference between the reactor walls and the substrate.
Another vertical growth cell configuration with improved uniformity is a “shower head” vertical CVD process shown in FIG. 3 for forming a material layer on a surface of a substrate with a shower head 22 directing the gaseous flow to contact the substrate 3, which has a substrate heater 11. The configuration is characterized in that (1) the surface of the substrate 3 is spaced an average distance less than ¼ the substrate effective radius from the shower head directing surface, (2) such directing surface is defined to have an incompressible imaginary sphere with a diameter of 1/10 the substrate effective radius along all accessible surfaces of the shower head and (3) at least 50 percent of the gas flow that contacts the substrate undergoes contact initially at an interior point of the substrate surface before it cuts a plane that is tangent to the periphery of the substrate and normal to the surface of the substrate upon which deposition is desired.
The main disadvantages of this shower head configuration is the parasitic deposition on reactor head and walls resulting from the close proximity of the shower head and the substrate and the difficulties in obtaining temperature difference between the reactor walls and the substrate.
There may be mentioned as prior art: U.S. Pat. No. 6,176,925; U.S. Pat. No. 6,177,292; U.S. Pat. No. 6,179,913; U.S. Pat. No. 6,350,666; U.S. Pat. No. 5,980,632; U.S. Pat. No. 6,086,673; U.S. Pat. No. 4,574,093; and “Handbook of Crystal Growth”, vol. 3, edited by D. T. J. Hurle, Elsevier Science 1994.